The aim of any risk management carried out on a site is to control the potential health and environmental risks to any receptors, whether they be site users, maintenance and/or excavation workers, or people off-site (eg, those affected by the migration of contaminants in dust, groundwater or surface water). Potential health risks arise from exposure to contaminated soil, groundwater or vapours. For an actual risk to occur, however, a complete exposure pathway must exist between the contaminant source and the receptor.
Remediation acts by reducing the concentrations of contaminants to such a level that exposure to the affected media (eg, soil) will not result in a significant risk to the relevant receptors. Remediation options include removing the contaminated soil to safe disposal (whether on- or off-site) or treating the soil to reduce the contaminant concentrations to safe levels.
Managing a contaminated site, on the other hand, implies controlling the exposure pathway and/or the behaviour of potential receptors. That is, although the hazardous contaminants may remain in the soil, the management controls prevent them from affecting the receptors. In situ management options include:
- implementing physical barriers such as fencing, placing a sufficiently thick soil or cement cap, or sealing the site with an impervious pavement (contaminant isolation)
- setting aside the site as reserve land (for which less frequent exposure can be expected and therefore less stringent guidelines can apply)
- implementing institutional controls such as management plans, or zoning the land to prevent sensitive uses.
In many instances, a combination of remediation and/or management approaches may be appropriate. For instance, where a soil cap is used to isolate users from affected soil (blocking the exposure pathway), a management plan should be implemented at the same time to control any excavations into the underlying affected soil (control on people's behaviour).
Remediation or management of a known disused sheep-dip site needs to be undertaken if a site investigation report identifies contaminant concentrations that pose a risk to human health or the environment. However, in some cases where contaminants are present at concentrations below guideline criteria but elevated above the background concentrations, an owner or developer may still choose to implement management or remediation options to reduce risk or allay adverse perceptions of potential purchasers. It is considered best practice for landowners to undertake remedial activities with the knowledge and agreement of territorial authorities.
The selection of a remediation or management option is very site-specific and depends on many factors, including the:
- type, extent and depth of contamination
- location of contamination relative to receptors
- area and volume of the contamination relative to the size of the property (or subdivision development) the sheep-dip site is part of
- proximity to a safe disposal location (including on-site disposal) if excavation and replacement are being contemplated
- availability of clean soil for capping or dilution
- future use of the site
- financial resources available.
Ideally, the option that best reduces the risk, maximises environmental merit and minimises costs would be chosen. Clearly, there are more options available for dealing with a small dip site in a large subdivision development than for a dip within a single residential lot, where removal or sealing over may be the only practical options.
A range of resource or discharge consents may be required for old sheep dips in relation to current off-site discharges, remediation activities, and the moving or transport of contaminated material, depending on the requirements of district and regional plans. Landowners should consult with both territorial authorities and regional councils to determine appropriate remediation options and consenting requirements. Territorial local authority officers should consult their regional council for guidance on regional plan requirements. In turn, regional councils should consult with the territorial local authority regarding what district plan provisions might be relevant.
Resource consent may be required when:
- the soil contamination, whether left undisturbed or subjected to some management or remediation action, is likely to cause an off-site discharge to groundwater or surface water through the action of rain infiltration or run-off
- remediation involves the transfer of soil to an area that is not a disposal facility consented to receive such material
- remediation involves disturbing a soil volume or area in excess of thresholds in the relevant plan
- remediation is likely to cause a discharge of contaminants to air (dust) or to water (as sediment in run-off).
In the case of land-use change or the subdivision of former farmland, the territorial authority needs to consider if resource consents may be required and should consult the relevant regional council at that time.
6.3 Remediation action plans
When site remediation is the chosen option, a local authority should advise the applicant to submit a remediation action plan as part of the resource consent application. The preparation of a remediation action plan is discussed in Contaminated Land Management Guidelines No. 1 (Ministry for the Environment 2003a). The identification of all potential physical and chemical hazards and steps that will be taken to eliminate, isolate or minimise these hazards should be part of the plan, to inform workers undertaking the remediation. The complexity of some situations is likely to require an owner or developer to employ specialist expertise when preparing remediation action plans. The local authority may seek independent review of the proposed plan(s) or advice from a contaminated site officer at the relevant regional council. Regardless of which method is proposed by a developer, the success of the remediation must be confirmed by validation sampling and/or ongoing monitoring, as outlined in sections 6.6 and 6.11.
A landowner or developer may initiate a site investigation and remediation voluntarily to minimise the risks from an old sheep-dip site on her or his land. Outside the resource consent process there are often no requirements for the landowner to follow best practice procedures or to inform the council about the course of action. To encourage the landowner to involve the council in the investigation and remediation, councils could provide a certificate of compliance upon successful site remediation free of charge. This would give assurances to both parties: the landowner has confirmation from the council that the land is not contaminated, and the council has confirmation that the criteria for a successful remediation were met.
6.4 Soil remediation options
Remediating a site involves undertaking work to reduce the concentrations of contaminants at the site. Sometimes the contaminant concentrations can be reduced to background levels for the area under consideration, but it is more usual for the concentrations to be reduced so that they comply with the guideline criteria or standard for the proposed land use. Care needs to be taken to ensure remediation goals are appropriate to the local situation, because some contaminants (such as arsenic) can naturally occur at concentrations above guideline criteria. The selection of remediation goals and guideline values should only be undertaken by experienced contaminated land professionals.
The degree of remediation necessary will vary from site to site. At one extreme, soil contamination may be limited to a small, discrete area (eg, immediately under the dip area, amounting to a few cubic metres of soil). In this case, it would be possible to complete a full remediation by removing all the affected soil and replacing it with clean soil. Provided the chemicals have not migrated down into groundwater, removal and replacement of the soil beneath and around the dip site would remove or reduce the contaminants to a level that is safe for people and the environment.
At the other end of the spectrum, a large dip site might have high concentrations of contaminants down to several metres over a small area under the dip, and moderate concentrations over a more extensive but shallow yard area. Such a situation might lend itself to a range of methods that result in only partial remediation, rendering a site suitable for its intended use, perhaps with the assistance of a management plan to ensure that any residual risks are accounted for and managed appropriately. For example, one remediation approach may involve a combination of digging out the highly contaminated soils down to a "safe" depth, with disposal off-site, and the remainder of the yard area made safe by soil treatment, such as sieving and backfilling the large soil particles and land-filling the fine particles.
The common combination of arsenic and organochlorines does limit the options for achieving remediation via an all-inclusive soil treatment. The most appropriate site remediation methods for disused sheep dips are likely to involve:
- the physical removal of contaminated materials and soil and their relocation to a more suitable area (on- site or off-site) with long-term management, or disposal to landfill
- soil treatment to reduce the contaminant concentration to safe levels.
A range of alternative options should be considered, and a selection of remediation options is provided below. The list is not exhaustive, and there may be more appropriate remediation techniques that can be applied at a particular site.
Excavation and replacement (dig and dump)
Excavation is commonly used where a detailed site investigation has identified significant contamination in the soil or unacceptable discharges to water bodies. For this remediation method the contaminated soil is removed to a licensed landfill, or to an area that has consents to receive contaminated material. The removed soils or sediments may need to be analysed to confirm they comply with landfill waste acceptance criteria as specified in the Hazardous Waste Guidelines (Ministry for the Environment 2004b). Where soils from sheep-dip sites fail the toxicity characteristic leaching procedure (TCLP) tests required by most landfills, prior treatment to reduce the contamination will be necessary. Concrete (eg, from the draining platform) can also adsorb dip chemicals, so where no risk management plan is to be instituted for a site, old dip-bath structures should be removed together with the soil.
Validation samples will need to be collected from the soil remaining at the site to prove that the contaminant concentrations comply with the remediation goals (see section 5.6). Remaining holes or depressions may then be covered with clean material. This option is practical in a farming context. In the past, private landowners often "decommissioned" their old sheep dips by doing the dig and dump themselves without supervision, often unaware of proper remediation procedures.
This option is likely to be economic mainly for small volumes of soil, but may also be appropriate for extensive contamination on large developments where the developer considers the economic benefits justify the expense.
Soil screening and soil washing
Soil screening is a process that attempts to physically separate the contaminated material from the rest of the soil. Physical separation is an ex situ process that requires soil excavation before treatment. It is an effective soil treatment for contaminants adsorbed to soil particles that occur in a particular size fraction of the soil. The majority of contaminated material is often contained in the finer clay and silt particles (< 63μm) because they bind contaminants strongly due to their large and reactive surface area. However, not all soils are amenable to this treatment and the potential application of this technology is usually ascertained by laboratory treatability tests and pilot scale tests. Soil screening and soil washing are often used to reduce the amount of material for subsequent treatment or disposal.
Screening separates soils according to particle size by passing the material through a sieve with a particular mesh size. Normally soil undergoes preliminary screening by separating large rocks and debris from the soil matrix. Any residual fines adhering to the surface of large rocks are washed off, and it is verified that the coarse fraction is clean before returning it to the site. During the subsequent screening process, finer soil particles pass through the sieve and leave larger particles behind. Screening may be performed as a stationary process or with motion.
Where the separation is not complete due to larger clay conglomerates or particle coatings (eg, metal oxide on particles in larger size fractions) dry sieving is often followed by wet sieving or soil washing to separate the fine fraction with the contaminants from the remainder of the soil matrix more effectively.
Soil washing involves adding water to the soil and then wet sieving the resulting slurry. Most soil washing processes employ secondary screening to segregate the particles into different-sized fractions, usually between 5 mm and 60 mm. After the contaminated fine particles are separated from the clean coarse particles, both fractions are dewatered with a filter press. The contaminated fraction and/or the process water is further treated or disposed of in a landfill, while the larger particle fraction can be returned to the site if considered clean. In general terms, soil washing is cost-effective for soils where the clay and silt contents are less than 30-35 percent of the soil (Pearl et al. 2006). Soil washing is suitable for removing both organochlorines and arsenic.
In situ biological treatment/bio-remediation
Modern dip chemicals will naturally break down due to biological activity in the soil over a period of weeks or months, provided the concentrations of historical and/or currently registered chemicals are not so high as to prevent biological activity. An appropriate strategy for modern dip chemicals is therefore to wait for natural attenuation to occur, taking samples at intervals to ensure the natural reduction in contaminant concentration is indeed happening. It may be appropriate to fence the site during this process, depending on its location and accessibility.
In contrast, some of the historical dip chemicals, such as the organochlorines DDT and dieldrin, break down very slowly, and the metalloid arsenic does not break down at all. Leaching of these chemicals out of the soil is also generally slow. Although bio-remediation of organochlorines is accelerated in strict anaerobic environments, such as found in black pond or drain sediments, most organochlorine pesticides at dip sites are normally found in the topsoil under mainly aerobic conditions. Thus, in the case of DDT and dieldrin, attenuation will occur slowly over years, while arsenic will attenuate only minimally.
Some contaminants are accumulated in the leaves of suitable plant species. Arsenic, for example, accumulates in high concentrations in water plants such as watercress. There are also a few land-based plants that, given the right conditions, can produce very high concentrations of arsenic in their leaves. The leaves then require harvesting and off-site disposal, otherwise they will drop on the ground where they break down and redeposit the arsenic. Such a remediation technique is slow (taking years, possibly decades) and requires consistent maintenance to lower concentrations significantly. This would only be feasible where a long-term gardening programme is in place, such as for a retirement village or a council reserve.
Research into phytoremediation in New Zealand is still in its early stages. Given that proposals require detailed technical evaluation, and due to their nature would require long-term monitoring and validation sampling, phytoremediation is unlikely to be viable for most sites at this time.
Electro-osmosis is the only in situ technique that can yield clean soil in relatively short timeframes (one to three years) when metals such as arsenic are the prime target. Electro-osmosis is the movement of water through soil by applying a low-intensity direct current between electrodes placed in the soil. Contaminants are mobilised in the form of charged particles, or ions, in the pore water. This technology works well in soils with poor permeability and at a depth where contamination in the hot spot areas is normally encountered. It is also very suited to the treatment of excavated soil, either on-site or off-site at a central treatment facility. On the down side, the technique requires rather intensive, and therefore costly, instrumentation and monitoring. Electro-osmosis is likely only to be suitable for sites where arsenic is the sole contaminant of concern and the extent of contamination is large enough for this in situ technique to be cost-effective compared with land-filling the contaminated soil.
In situ soil mixing
In some circumstances, it is possible to reduce contaminant concentrations to below guideline concentrations by vertically mixing the contaminated soil with underlying uncontaminated soil; in other words, by diluting the contaminant concentrations on the surface. Soil mixing should not be used for hot spots, and should be limited to contaminant concentrations less than two to three times the guideline level. For example, for large yard areas where contamination only marginally exceeds soil guideline values and is probably restricted to near the surface, soil mixing may be practical provided there is sufficient clean underlying soil available for mixing and no potentially contaminated dip structure remaining in the ground. Mixing can be achieved by using a large, sturdy rotary hoe, or by repeated scarifying, windrowing and re-spreading with a grader. Strict controls need to be in place to manage sediment run-off during soil mixing and to avoid contaminating the wider environment.
There are practical difficulties for sheep-dip remediation when attempting to mix depths of soil greater than about 30-40 cm. If there is any deeper contamination, such as under the dip, then excavation will be necessary to achieve mixing, in which case it is probably just as practical (and certainly more desirable) to remove the material from the site.
There are other practical limitations of soil mixing.
- The soil needs to have a suitable texture and moisture content to break down to a fine state when cultivated ie, the soil must be intimately mixed together to achieve homogeneity throughout the profile, rather than just mixing soil clods or clumps together. Any clay-rich soils will need extensive drying and grinding to allow good mixing, and repeated scarifying in dry weather over sufficient time to allow the lumps to disintegrate. Successful mixing is difficult to achieve with heavy clay soils, whereas silt loams and sandy soils are more readily mixed.
- There must be a sufficient depth of clean soil underlying the contaminated soil to create a composite soil. A thin top soil overlying large gravels, or heavy clay, is unlikely to provide a satisfactory mixed soil.
Field-mixing trials need to demonstrate that the soil can be adequately broken down and mixed, and the results need to show consistent lateral and horizontal distribution. They should be checked independently by the local authority, either by trained council staff or by a different contaminated site professional from the one involved in the remediation process.
In situ soil mixing is a rather controversial remediation method. Although the overall concentration of contaminants is reduced, it leads to a larger amount of soil being contaminated, which is not a desirable option. On the other hand, this method could be considered as a sort of resource recovery to achieve beneficial use of the soil rather than creating soil waste to go to landfill. Vertical soil mixing is practised in Australia for agricultural land that is being redeveloped for residential use. The New South Wales Environment Protection Authority (NSW EPA) has issued guidelines which describe the application and limitations in more detail (see http://www.epa.nsw.gov.au/resources/vertmix.pdf). However, with the high natural background concentration of arsenic in New Zealand, the efficiency of mixing may be greatly reduced.
6.5 Management of discharges during earthworks
Earthworks for remediating dip sites will generally be at the smaller end of the scale, and so will be the potential for off-site discharges. Discharges of contaminated dust during windy conditions are most likely to cause nuisance issues for neighbours as opposed to health issues, unless the soil is heavily contaminated. Discharges of contaminated sediment during heavy rainfall and tracking of contaminated material off-site by vehicles are possible. Significant discharges to groundwater as a result of earthworks are generally unlikely, but good practice should be followed to avoid any discharges.
Four basic best practice principles are:
- stage the excavation
- stabilise exposed areas rapidly
- install perimeter controls to divert stormwater
- employ retention devices.
Staging the excavation involves minimising the area of soil exposed, so that the opportunity for sediment or dust generation is minimised. For small excavations, the most effective strategy is to carry out all excavation during dry, calm conditions. The smallest remediation projects can be carried out in a single day. Otherwise, loose soil should be protected from erosion by stabilising exposed areas rapidly. This will generally involve compacting at the end of each day, and covering stockpiles of contaminated material in adverse conditions.
Perimeter controls such as diversion drains should be installed above the site to keep clean run-off out of the work area during the excavation process. Perimeter controls can also retain or direct sediment-laden run-off within the site; typically this will involve diversion drains, silt fences and earth bunds. Retention devices such as settling ponds may be necessary. Water collected in the excavated area, either from rain or other sources, should be treated as potentially contaminated. Before disposal it should be tested and then disposed of in an appropriate manner. In the case of vehicles leaving sites, wheels should be cleaned, soil should not be piled so high that it might spill, and loads should be covered and sealed to prevent spillage and dust. Guidance on controls for earthworks is given in regional council guidelines, such as ARC 1999.
Health and safety considerations are also an important part of any contaminated site investigation or clean-up. The risks − such as toxic effects, physical injury and harm to workers on-site − must be assessed and managed. Where the soil is heavily contaminated, the duration and extent of exposure may be important in determining whether a human health impact is likely. In this case, protection of on-site workers is also relevant. For disused sheep-dip sites it is likely that exposure would only occur over a short period of time. For example, to avoid operators in open excavators or bulldozers being exposed to clouds of potentially contaminated dust, it is recommended that earthmoving machinery have pressurised cabins with GAC filters on air inlets. Occupational health and safety requirements for field works are covered under the Health and Safety in Employment Act 1992 (see Appendix 3 for further information).
6.6 Site validation investigation
Whenever remedial action is taking place, validation sampling must be carried out to determine whether the clean-up goals have been achieved. Validation sampling should preferably be carried out by an independent party (ie, independent from the party who carried out the investigation and the remedial design).
The validation has to be undertaken with a sharp eye for unidentified contamination or possible missed hot spots. Therefore, the investigator should not only verify the remediation works systematically, but should also validate the remaining soil and/or groundwater, and the backfill soil if applicable. While often carried out together, validation sampling and verification of remediation goals are two different activities.
Samples are then analysed for selected chemicals of concern. Normally a single chemical is selected from those previously analysed at a site, which will be the contaminant of concern that exceeded its guideline more often and by a greater amount than any other substance. It is expected that any other chemicals would automatically be below their respective guidelines if the critical chemical is below its guideline. However, in the context of old sheep dips it is recommended not to substitute dieldrin and arsenic for each other because they may have a different mobility in the soil profile due to their physicochemical characteristics.
In the case of soil excavation, the base and sides of the excavation should be sampled to ensure a sufficient volume of soil has been removed for the remaining soil to be below guideline concentrations. The number of samples will depend on the size of the excavation. For example, for a small excavation the samples should include at least one sample from each of the base and four walls of the excavation. In the case of soil mixing, validation samples must be taken from the mixed soil and the soil immediately below the mixing zone to show that both the underlying soil and the mixed soil are below guideline values for the contaminants of concern.
The number of validation samples proposed and the target concentrations should be specified in the remediation action plan. More information on validation sampling can be found under Contaminated Land Management Guidelines No. 5: Site Investigation and Analysis of Soils (Ministry for the Environment 2004a).
6.7 Site management options and strategies
The management of a contaminated site limits the exposure of people or environmental receptors to a hazardous substance by controlling access and contact to the contaminant. The two main options are:
- imposing a physical barrier to isolate the contaminated material and prevent exposure (eg, in situ burial and/or capping)
- finding an alternative lower-risk land use.
Contaminant isolation involves the use of an effective barrier to prevent people getting in contact with the contaminated site. An effective barrier is one that blocks the exposure pathway between a contamination source and a receptor. Barrier types include fencing to prevent uncontrolled access to an affected site, and sealing or capping an affected area. Discharges may still occur, however, and so these options may require a discharge permit.
When considering site management options for an identified sheep-dip site, it is important to take the current and proposed land use into account. For ongoing farming practices, capping may be the most practical option, while capping as a remediation option in residential areas (including schools) is more problematic, because it is difficult to ensure the cap is not dug up again.
Fencing a site may be appropriate in a rural location where few people reside and control of the site can be readily exercised by the property owner. This option may be suitable for land where the location of a sheep-dip site is known and where the land is to continue in agricultural or low-intensity use.
Fencing is less satisfactory in a residential area as curious children are more likely to enter the site. Here, an additional means of preventing exposure such as capping is recommended, and child-proof fences in residential areas should consist of chain-link netting at least 2 m high. Permanent signs should be erected, or attached to the fence at clearly visible places, to warn the public of the contamination. Placing the site on the LIM would help to ensure that fencing is maintained. The council may instigate a site visit routine to check that warning signs and fencing are still in good order. Fenced areas ideally need some form of maintenance-free vegetation cover to avoid erosion or slips. Bushy or prickly plants may act as a further deterrent to children entering the site.
Capping is usually only done where the contaminant concentrations are not higher than a few times the guideline concentration. There are two main forms of capping a former sheep-dip site: soil capping, and sealing using asphalt or concrete. Soil caps with a low-permeability base layer are common, cheap to install and, when vegetated, easy to maintain. A soil cap must be of sufficient thickness that excavation through the cap would be rare, and should involve using something like a geotextile membrane to separate clean from contaminated soil. Asphalt, concrete, or buildings directly block a receptor from contact with the ground. Low-permeability sealing materials can also be used to prevent water getting in, thereby preventing leaching of mobile contaminants into the groundwater system.
In a rural situation where the owner can exercise additional control over access, a soil or impervious gravel capping layer of an appropriate depth could be satisfactory. The thickness of the capping layer should be determined to provide sufficient depth for grass to be established without the roots penetrating the contaminated soil. If there are several disused sheep dips on one farm property, it may be an option for the farmer to excavate them all and encapsulate the contaminated soil material from the various sites in one spot. However, this would require a resource consent from the regional council unless specifically allowed by a rule in the regional plan.
Soil capping within residential subdivisions is generally not advised because it is not possible to control what future homeowners and tenants will do in their backyards. If capping in a residential situation is done regardless, soil caps of at least 600 mm and up to 1 m, depending on the contaminant concentrations, may be necessary to ensure minimal access. A geotextile separation and marker layer should be placed between the contaminated soil and the soil cap. On reserve land used for passive recreation, a capping layer of 400 mm may be sufficient, where cultivation or excavation, other than initially to plant grass, is unlikely. Plants planted in capped areas need to be selected with care as deeper-rooted plants (eg, trees) may disturb the cap.
Excavation and on-site disposal
Proposals for on-site disposal require evidence that the disposal site will not create adverse effects or present risks to potential receptors. In general, chemicals from old sheep dips are not particularly mobile in the environment, so on-site disposal away from water courses and above the water table may be an effective solution. A resource consent may be required, as described in section 6.2.
Moderately contaminated material could be disposed of on-site to a passive recreational reserve, with an appropriate layer of clean soil cover to ensure contact with contaminated soil is unlikely. A geotextile layer would normally be placed above the contaminated soil to separate it from the clean capping layer and to act as a warning in the event of excavation. A management plan would be implemented to control excavation. Development of the reserve may be prohibited by means of a notice on the title, or the LIM.
Highly contaminated material (concentrations several times the guideline values, and with a high potential for leaching) would need to be encapsulated on all sides if on-site disposal was contemplated. Often the soil is treated with a binder such as cement for stabilisation or solidification before it is placed in an engineered impermeable cell. This option would require detailed technical proposals.
Alternative land use
Finding an alternative use for the affected area means a less sensitive land use for which higher contaminant concentrations can be tolerated, because people will experience less frequent exposure. Such uses include:
- reserve areas, where people generally spend less time compared with the residential land-use scenario, and hence the exposure is limited
- under roads, where management plans are in place to ensure that no casual exposure to ground contaminants can occur, and to control any excavations.
Note, however, that topsoil contaminated with pesticides often has high humus levels and is difficult to compact, which precludes its use as road base. Caution is also required with respect to the proximity to services such as water/sewer pipes, telecommunications and electricity lines, and access for maintenance workers. The use of concrete U-ducts would allow future modifications to cabling or pipes without frequent re-excavation.
6.8 Risk management plans
Risk management plans contain information about the contamination on a site. This information can then be easily transferred to new property owners or site users. Management plans are designed to avoid people, stock or other environmental receptors coming into contact with the contaminant (eg, by imposing a physical barrier). Risk management plans are mainly employed to control excavation activities and to ensure the contamination is kept securely contained. They may address caps on slopes that are prone to slips and erosion. Some management plans also specify ongoing monitoring requirements. The contents of a management plan are discussed in Contaminated Land Management Guidelines No. 1 (Ministry for the Environment 2003a).
One limitation of a risk management plan is that it relies on a responsible party being given the authority to control a site. A risk management plan is therefore most appropriate where a corporate body (eg, a company or a local authority) is in control of the land. Management of a contaminated site may also be appropriate for a bigger parcel of land or a lifestyle block. For owner−occupier residential sites, a risk management plan may be imposed on a subdivision by way of a consent notice on the certificate of title, or as a condition on a land-use consent as an ongoing requirement. Risk management plans should have a review clause to ensure the plan continues to provide adequate protection of human health and the environment in the future.
As a protection to future owners, any site that is not returned to a fully remediated state, and thereby still falls under the definition of contaminated land, should be recorded in council files and appropriate information should appear on property files or LIMs/PIMs. Private landowners should always be informed if such a notice is put on the property title. If a risk management plan is in place, it is the primary responsibility of the site owner to make it available to site users, where appropriate.
6.9 Contamination below existing barriers
Where contamination exists (or is thought to exist) below previously constructed barriers, such as woolsheds, these barriers can effectively cap the contamination and preclude exposure to contaminated soils (NSW Agriculture 1996, p 26). Anecdotal information suggests, however, that in some cases the dip liquid was purposefully disposed of under the woolshed in order to get rid of insects.
There are two options to deal with this situation:
- remove the contamination, which would involve destroying the associated structure, or
- leave the contamination in place and put a notice on the title or LIM as to the possible presence of contamination, coupled with an appropriate risk management plan.
6.10 Groundwater contamination
In most dips, contaminated soil has been exposed to similar conditions over the past 40 years or more, and chemical reactions between soil and contaminant are therefore expected to have reached some form of equilibrium. In other words, most of the contaminants that would leach under present circumstances have probably already done so (Dupen et al 1994, p 17). To ensure that groundwater has not been significantly affected, it is recommended that the groundwater below a dip site always be assessed by a technical expert unless it can be proven that a minimum of three metres of unfractured clean soil or low-permeable solid rock separates the contaminated zone from the water table.
Investigations are also warranted when one or more of the following conditions apply:
- the site is close to a stream or lake, which is the receiving water for groundwater passing under the site
- the site is close to a water-supply well, the most likely situation being a farm bore
- the concentration of dipping chemicals used on the site is very high
If groundwater contamination is found, the follow-up investigations require detailed consideration of the impacts and possible remediation measures. This is beyond the scope of this guideline. In general, source removal (ie, soil excavation) will reduce future impacts on groundwater, and improvements could be rapid (a matter of months) where ground permeability is high.
Appendix 7 contains a case study where groundwater was investigated for arsenic contamination in relation to suspected contaminant leaching from old dip sites.
Most on-site or in situ remediation involves careful monitoring of the remedial system, the remediation processes and the effects of the processes. Monitoring of the effects (eg, measures of dissolved oxygen for aerobic processes, the distribution of added substances, etc) can give an early warning that the clean-up level will not be reached if the system or process keeps running as it is, which allows the process to be corrected or a change of contractor/consultant made.
Verification monitoring (third party) focuses on the reduction of contaminants achieved. In practice, verification monitoring often provides data when most of the money has been spent and the remediation is completed. Due to the time lag between actual remediation and receiving the sampling results, verification monitoring is not a good tool to assess the progress of remedial processes. For this reason, it is recommended that larger remediation projects not rely on verification monitoring alone, but also monitor the remedial system, the processes or effects during the remediation phase.
Environmental monitoring may be appropriate if the site is of sufficient scale that effects on groundwater or surface water are likely, or during remedial earthworks where run-off may affect surface water. If these circumstances arise, it is likely that resource consents for the discharge to water will be required, which provides the opportunity to impose monitoring conditions.
Always give careful consideration to the need for ongoing monitoring, especially if capping or fencing is chosen to manage the site, in order to ensure that barriers are still intact and in place. Monitoring conditions should not be imposed automatically though, and each case must be treated on its merits. For example, if a dip site is particularly large and concentrations are significantly elevated, or a drinking-water supply is at risk, a requirement to investigate and monitor groundwater will be necessary. Monitoring of groundwater in sedimentary areas, where groundwater is expected between three and five metres, could require the installation of groundwater monitoring wells to about 5-7 metres.
Surface-water monitoring may be appropriate if there is a nearby water body and the site layout and topography are conducive to sediment run-off reaching it. Taking sediment samples at suspected points of entry at regular intervals is an effective way to determine if there are any remaining risks to the environment.